Contrast imaging has been conducted by X-ray CT (Computed Tomography) scanner, which requires injection of a contrast agent into an object. In this contrast imaging, it is required in advance to decide an acquisition time at which an imaging scan is performed after injecting the contrast agent. For such a decision, changes in the CT value within a ROI (Region Of Interest) are monitored contrast agent injected into an object. The imaging scan is started at a time when the CT value within the ROI has reached a predetermined threshold. That is, this time is used as triggering timing for the imaging scan.